In the printing industry, it is sometimes necessary to process media multiple times in order to create a final product. For example, in order to implement duplex (e.g., two-sided) printing in high speed printers, media such as paper may have a first side printed at a first print engine and the second side printed at a second print engine. In other instances, a digital printer may not have enough capability (e.g., an ink jet printer may not be able to print Magnetic Ink Character Recognition (MICR), or a monochrome printer may not be able to print color/highlight color). In these cases, it may be necessary to process the media through multiple, independent digital printing devices in order to achieve the final product.
It some instances, a vacuum unit may be inserted between the printing engines in order to improve stability of the paper web. Moreover, utilizing the vacuum unit in conjunction with a cooling tower improves paper cooling by reducing the paper temperature to a significantly greater extent than conventional air cooling. However during duplex print operations, a print void may form on the second side (e.g., side 2) of the paper located within the vacuum unit whenever the paper web is stopped between printing devices.
Particularly, a side 2 void at the vacuum unit is created when hot paper loses or absorbs moisture in a non-uniform way. For instance, the paper is typically very hot from fusing (e.g., having been heated by the preheat platens and hot roll) upon exiting the first print engine. Thus, the vacuum unit metal rollers and metal sheet are hot from having absorbed heat from the moving paper web. Whenever the web motion is stopped, the paper remains in contact with the hot rollers and sheet in the vacuum unit.
While sitting on a hot roll the paper will absorb moisture at a much different rate than paper floating in air. However, when paper is being pulled down on a hot roll, moisture is absorbed on the sides of the paper, while the heat of the roll forces out the remaining moisture in the contact region of the paper. Around this contact area, paper is free standing in air and not in contact with any surfaces. The hot paper may absorb moisture in these airborne regions and cool off quickly compared to the regions sitting on the hot rollers and metal plates.
Losing moisture and being in contact with hot surfaces causes paper to shrink in the contact regions, while the free standing paper does not experience this shrinkage. In the transition or boundary area between heated/shrunk paper and free standing paper, the paper experiences a surface distortion (wrinkling, mottle, texturing, waviness, etc.). This distorted area, when passed through a second print engine in a duplex system, does not allow for good toner/ink transfer or fusing and side 2 voids are created. As a result, an undesirable loss of print data and blank regions in side 2 of the print output may occur.
One way to prevent side 2 voids is to avoid the use of a vacuum unit. However, using no vacuum unit at all would result in instability in the paper path, thus reducing printer performance and fewer crashes. Another solution is to use a muffin fan type of loop stabilizer (e.g., a Lasermax design). Nonetheless, muffin fan units do not stabilize the paper web and are very susceptible to outside air disturbances. Further, muffin fan units do not provide the additional cooling obtainable with vacuum units.
Accordingly, a mechanism for preventing vacuum unit paper voids during web stoppage in a printing system is desired.